Attention has been heretofore paid to utilization of an excimer laser light unit to serve as a light source for an unit for projecting and exposing an image on a reduced scale (hereinafter referred to as a stepper) for the purpose of producing semiconductor devices. This is because of the fact that the excimer laser light unit has many excellent advantages that it is possible to expand a limit of light exposure to the range shorter than 0.5 micron because the wavelength of an excimer laser light is short (about 248.4 nm in the case of a KrF laser light), the excimer laser light has a deep focus depth compared with a g line and an i line generated by a conventional mercury lamp under a condition of same resolvability, a small number of lens apertures (NA) is required, the light exposure range can be enlarged, and a large magnitude of power can be produced with the excimer laser light unit.
In a case where the excimer laser light unit is used as a light source for the stepper, it is required that a laser light to be outputted from the excimer laser light unit is oscillated within the narrow-band oscillatory range. In addition, it is required that the wavelength of the output laser light from the excimer laser light unit which has been oscillated within the narrow-band oscillatory range is stabilized while it is controlled at a high accuracy.
Hitherto, a monitor etalon has been used for the purpose of measuring the wavelength line width of a laser light outputted from a narrow-band oscillating excimer laser light unit or the like, and moreover, detecting the wavelength of the same. The monitor etalon is constructed in the form of an air gap etalon including a pair of partially reflecting mirrors arranged opposite to each other with a predetermined gap therebetween. With such construction, the wavelength of the laser light which has permeated through the air gap etalon is represented by the following equation. EQU m.lambda.=2nd.multidot.cos .theta.
where m designates an integral, d designates a distance between the opposing pair of partially reflecting mirrors constituting the air gap etalon, n designates a refractivity as measured between the opposing pair of partially reflecting mirrors, and .theta. designates an angle defined by a normal line of the etalon and an optical axis of an incident light.
With respect to the foregoing equation, it is obvious that .theta. varies as the wavelength of the laser light varies, when it is assumed that n, d and are kept constant, respectively. In practice, the wavelength of a light to be detected is practically detected by utilizing the aforementioned nature of the monitor etalon. With the monitor etalon constructed in the above-described manner, however, the above angle .theta. varies as a pressure in the air gap and an environmental temperature vary, even though the wavelength of a light to be detected is kept constant. In view of the above fact, when the monitor etalon is used for performing a detecting operation, the wavelength of the light to be detected is practically detected while the pressure in the air gap and the environmental temperature are controllably kept constant.
However, since it is practically difficult to control the pressure in the air gap and the environmental temperature at a high accuracy, the absolute wavelength of the light to be detected can not be detected at a sufficiently high accuracy.
To obviate the foregoing malfunction, a proposal has been made with respect to an apparatus for detecting the absolute wavelength of a light to be detected by inputting the light to be detected as well as a reference light having a known wavelength (e.g., an argon laser light, an oscillation line derived from an iron, an oscillation line generated by a mercury lamp or the like) into a monitor etalon and then detecting a relative wavelength of the light to be detected relative to the reference light.
With this proposed apparatus, the light which has permeated through the etalon is irradiated directly toward the detecting surface of an optical detector such as a CCD image sensor or the like to form an interference fringe on a detecting surface of the optical sensor so that the absolute wavelength of the light to be detected is practically detected based on the position where the interference fringe is detected.
It should be added that a technology using a diffractive grating in place of the monitor etalon is available at present.
However, in a case where a light source for a reference light is arranged independent of a light source for a light to be detected, the absolute wavelength of the light to be detected can not practically be detected at a high accuracy by utilizing the foregoing technology. In detail, when the light source for the reference light is arranged independent of the light to be detected in the above-described manner, there is a possibility that the wavelength of the reference light inputted into a monitor etalon is largely deviated from the wavelength of the light to be detected, e.g., an excimer laser light. For this reason, the absolute wavelength of the light to be detected can not be detected a high accuracy. In addition, also in a case where the wavelength of the reference light is nearly equal to the wavelength of the excimer laser light, it is practically difficult to detect the absolute wavelength of the excimer laser light at a high accuracy when an intensity of the reference light is weak and a wavelength line width of the reference light is widened.
To obviate the above malfunctions, a proposal has been made such that a 253.7 nm oscillation line generated by a low pressure mercury lamp (having a natural mercury vapor enveloped therein) is used as a reference light because a wavelength of 253.7 nm of the above oscillation line is nearly equal to a wavelength of 248.4 nm of a KrF excimer laser light, and moreover, the low pressure mercury lamp has a high light intensity. However, it has been found that this proposal has drawbacks that a wavelength line width of the 253.7 nm oscillation line is wide and the absolute wavelength of a light to be detected can not practically be detected at a high accuracy due to a degraded detection accuracy for detecting each interference fringe.
The present invention has been made in consideration of the foregoing background and its object resides in providing a wavelength detecting apparatus which assures that the absolute wavelength of a light to be detected can practically be detected at a high accuracy by detecting an interference fringe derived from a reference light at a high accuracy wherein an oscillation line obtained from the vapor of a specific element such as a mercury or the like containing plural isotopes is used to serve as the foregoing reference light.